1. Field of the Invention
The present invention relates to a testing apparatus and a method for determining the specific work of debonding and/or cohesive failure of paints and adhesives.
2. Description of the Prior Art
For many years those skilled in the art have attempted to find a suitable method for measuring the adhesive (or debonding) strength of paint which would provide results independent of the testing process. One such process is a so-called "blister" method. In this process a fluid, either gas or liquid, is injected under a coating of paint through a hole in a substrate forcing the coating to lift from the substrate in the form of a blister. One example of testing using a "blister" method is described in an article entitled, "Measurement of Adhesion by a Blister Method", published in the Journal of Applied Polymer Science, Vol. A, Issue No. 14, pp. 125-134, 1961. However, none of the known blister testing methods has been widely accepted to evaluate the debonding strength of coatings.
Various other tests have also been devised to test the adhesive properties of both paints and adhesives. The American Society of Testing Materials through its Committees D-1 and D-14 has incorporated into its ASTM Standards some of these various test procedures to evaluate the strength of paints and adhesives. However, none of these tests as yet can determine the important adhesive strength property of paints and adhesives, which is the specific work of debonding. This is a parameter that is independent of the testing process. It is a function of only the paint-substrate and adhesive-adherend combinations tested, and the surface treatment of the bond areas.
I previously developed certain testing techniques for determining the specific work of debonding. These techniques are described in my prior U.S. Pat. Nos. 5,575,868 and 5,768,936. From repeated tests, I have since discovered that the debonding zones of those systems were often noncircular, and hence the calculations of the specific work of debonding using those systems is sometimes inaccurate. However, I have now invented and tested a new system which produces uniform debonding in both paints and adhesives. This new system results in being able to calculate accurate values of the adhesive strength or internal cohesive strength of both adhesives and paints. The failure of the test substance may occur as either a failure of the adhesive bond between the test substance and the substrate employed, or as an internal cohesive failure within the substance being tested itself. In either event, the adhesive or cohesive strength is expressed as the specific work of debonding or cohesive failure.